Notes for Authors

Studies on bond and atomic valences. I. correlation between bond valence and bond angles in Sb^III chalcogen compounds: the influence of lone-electron pairs

A modification of the present bond-valence model concept is suggested for compounds with Sb^III-X bonds (X = S, Se), which considers the influence of the lone-electron pair of Sb. A method to calculate a non-integer effective valence of Sb^III from the stereochemistry of ψ-[Sb^IIIX_n] polyhedra is derived.

The compositional dependence of octahedral tilting in orthorhombic and tetragonal perovskites

The angles of tilt of the BX₆ octahedra in orthorhombic perovskites, ABX₃, are directly related to the ratio of the volumes of AX₁₂ and BX₆ polyhedra. This relationship is investigated for known structures of oxides, fluorides and hydrides.

Structure of ζ-phase plutonium–uranium

The structure of the ζ-phase in the Pu-U system has been determined by neutron powder diffraction. The phase is rhombohedral with 58 atoms in the primitive unit cell.

Modulated structure of Bi₂Sr₂CaCu₂O_8+δ, a high-T_c superconductor with monoclinic symmetry

A refinement was performed on single-crystal X-ray diffraction data in a ninefold supercell. The monoclinic symmetry corresponds to a systematic shift of the modulation waves of consecutive slabs by π/9 with respect to orthorhombic Bi₂Sr₂Ca-Cu₂O_8+δ. Additional oxygen sites were detected in the Bi-O chains.

Structure of NaCd₂: an alternative path to a trial structure

A trial structure determination is described based on the initial assumption of tetrahedral close-packing. Only space groups, unit-cell dimensions and metallic radii are needed for determination.

Neutron and X-ray diffuse scattering of calcium-stabilized zirconia

Combined neutron and X-ray diffuse data were used to analyse the static disorder in CaO-stabilized `cubic' zirconia.

Neutron and X-ray diffuse scattering of calcium-stabilized zirconia at temperatures up to 1500 K

Diffuse neutron as well as X-ray intensities of calcium-stabilized zirconia can be described by a correlated distribution of the two types of microdomains. An anomolous intensity drop between 1250 and 1300 K may be related to a loss of correlated domains and the formation of free oxygen vacancies.

Structure of ferroelectric Pb₅Al₃F₁₉ at 160 K, polarization reversal and relationship to ferroelectric Pb₅Cr₃F₁₉ at 295 K

Pb₅Al₃F₁₉ undergoes a phase transition from ferroelectric to antiferroelectric at 270 K on heating. The atomic displacements from the ferroelectric structure to a hypothetical paraelectric structure led to a predicted Curie temperature of 860 (230) K.

Determination of the centroid or `the best centre' of a coordination polyhedron

A method for the calculation of the point in a coordination polyhedron with minimal variation of distances to the vertices is described. Its application in the analysis of irregular coordination polyhedra is proposed.

Interlayer interactions in M(OH)₂: a neutron diffraction study of Mg(OH)₂

Modeling of the H-atom thermal motion by anharmonic coefficients and by a three-site split-atom model. Comparison of interlayer interactions in Mg(OH)₂ and Ca(OH)₂.

Rietveld structure refinements of calcium hydroxylapatite containing magnesium

The crystal structures of four hydroxylapatite samples of different magnesium content have been investigated by X-ray powder pattern fitting. The results of the different refinement procedures indicate that apatite structure can host at most 10 Mg-atom-%, which is preferentially located in one of the two crystallographically independent sites.

X-ray diffraction study of Nd₂CuO₄ single crystals at 20 K

The structure of Nd₂CuO₄ at 20 K has been refined and compared with the room-temperature structure. The Gram-Charlier temperature-factor formalism was used to describe the Nd-atom displacements.

Incommensurately modulated structure of TaGe_0.354Te₂: application of crenel functions

The incommensurately modulated structure of TaGe_0.354Te₂ was determined by single-crystal X-ray diffraction, using crenel functions in the refinement in combination with an orthogonalization procedure.

Hydrogen bonding in α-ferrocenyl alcohols: structures of 1-ferrocenylethanol, 1-ferrocenyl-2-phenylethanol, 1-ferrocenyl-1-phenylpropan-1-ol, 1-ferrocenyl-1-phenyl-2-methylpropan-1-ol, 1-ferrocenyl-1-phenyl-2,2-di­methylpropan-1-ol, 1-ferrocenyl-1,2-di­phenylethanol and diferrocenyl(phenyl)methanol

α-Ferrocenyl alcohols, FcCH(OH)R and FcCPh(OH)R, exhibit a range of hydrogen-bonding patterns, including the formation of spiral chains and closed R₂²(4) dimers, as well as monomers containing intramolecular O-H⋯π(C₅H₅) interactions.

First sphere coordination of divalent metal cations by cyclodextrin: structure of the β-cyclodextrin–calcium chloride–water (1/2/11.25) compound

The presence of a Ca²⁺ salt in a β-cyclodextrin lattice is characterized by direct interactions of the cations with cyclodextrins and no contacts between cations and anions.

Single-crystal structure analysis and energy minimizations of a MFI-type zeolite at low p-di­chloro­benzene sorbate loading

The p-dichlorobenzene (pdcb)/H-ZSM-5 single crystal studied contains 2.56 (2) pdcb molecules per unit cell at the intersection of channels. Energy calculations support this interpretation.

The location of p-di­chloro­benzene in a single crystal of zeolite H-ZSM-5 at high sorbate loading

The zeolite H-ZSM-5 has been loaded with eight molecules of p-dichlorobenzene per unit cell. The structural details are comparable to those in the high-loaded H-ZSM-5/8p-xylene complex.

Molecular and crystal structure of azadirachtin-H

Azadirachtin-H, unlike other members of this family of C-seco-limonoids, has no carbomethoxy group at C(11), has a cyclic hemiacetal function at C(11) and has stereochemistry at C(11) opposite to that reported earlier based on NMR data. The molecule crystallizes in space group I4 with disordered ethyl acetate solvate.

Structures and photochemistry of 1,5-disubstituted dibenzobarrelenes

The effect of 1,5-disubstitution on the photochemistry of 9,10-ethenoanthracene-11,12-diesters has been studied and correlated with the crystal structures determined for several derivatives. Relationships are sought between the ratios of the two regioisomeric photoproducts and molecular and crystal structures; both electronic and steric effects are considered, but no one effect seems to dominate.

Electron-density study of m-nitrophenol in the orthorhombic structure

The electrostatic properties and the nature of the electron charge transfer of the m-nitrophenol molecule in the orthorhombic structure are discussed.

Predicting the crystal structure of organic molecular materials

This paper describes a novel method for predicting the crystal structure of organic molecular materials which employs a series of successive approximations to focus on structures of high probability, without resorting to brute force search and energy minimization of all possible structures.

Conformational analysis of acetylcholine and related choline esters

Conformationally dependent changes of the geometry of choline esters are elucidated based on results obtained from X-ray crystallographic studies and molecular mechanics calculations.

Interesting conformational and substitutional disorder in the crystal structures of three homologous crowns

The structures of three diphenylamine-containing crowns are reported and compared. An interesting substitutional disorder due to conformational flexibility is analysed.

Polymorphism of 7-di­methylamino­cyclo­penta[c]coumarin: packing analysis and generation of trial crystal structures

Polymorphism and computer generation of crystal structure from molecular structure for organic compounds are analysed.

Solid-state structure of a layered hydrogen-bonded salt: guanidinium 5-benzoyl-4-hy­droxy-2-methoxybenzene­sulfonate methanol solvate

The title compound crystallizes into a layered structure containing two-dimensional pseudohexagonal hydrogen-bonded networks.

Structural properties of synthetic ionophore GL₂E₄ diastereomers, 18-membered cyclic lactone tetraepoxides, by X-ray crystal analyses: possible relation between crystal structure and Ca²⁺-transportation ability. Addendum

Addendum to Acta Cryst. (1995), B51, 1045-1050.